SPB (Shortest Path Bridging), an Ethernet standard defined in IEEE802.1aq, is an extension of MSTP (Multiple Spanning Tree Protocol) aiming at constructing a large-scale non-blocking flat layer-2 network. SPB adopts SPB-IS-IS (Intermediate System to Intermediate System) to share link states in the network. Nodes in the network calculate the shortest paths between the nodes in parallel to avoid instability and low utilization ratio of some links resulting from the Spanning Tree Protocol (STP).
IEEE802.1aq defines two SPB modes, i.e., a VLAN mode (SPBV (SPB VLAN Mode)) and an M-in-M (Mac-in-Mac) mode (SPB MAC Mode). The SPBM (Shortest Path Bridging-MAC) mode inherits a packet encapsulation format defined in IEEE 802.1 ah and the multi-instance concept, and defines a new control plane and a new forwarding scheme for the data plane. SPB-IS-IS is used for studying link state information, synchronizing the link state information throughout the network, and calculating forwarding paths. SPBM is a technique for layer-2 Virtual Private Networks) (VPN), and has a network model similar to the M-in-M network model defined in IEEE 802.1ah.